I use oil based size to fasten the gold to the signs surface. It comes in slow or fast determined by the time it takes to tack up or dry to the point you can lay the gold. I like fast size for surface gilding as our shop tends to be dusty. The size looks like varnish and is clear. A thimble full was more than enough to do this project. A little goes a long way! We recycle plastic pudding cups for this type of job. Once we are done they are tossed and in that fashion I don't have to worry about using any solvents - except to clean my brush.
I brush the size on with a small brush. Because we routed the bevelled letters with a slight shoulder, raising them off the surface, they are pretty easy and fast to paint. Skill helps but isn't critical. A good, quality brush is critical however for a good job. I took my time and covered each letter well so there would be no holidays (missed spots) later.
Now comes the hard part... waiting for the size to tack up. How long we wait depends on the temperature, the humidity and if there is air movement. When its ready a knuckle dragged over the size will squeak. (do this on a test piece - not your finished work)
Next installment we'll get to the flashy part...